Hypercalcemia frequently accompanies malignancies, accounting for significant mortality and morbidity in these patients. The mechanisms responsible for this syndrome, however, are largely unknown. We have identified a potent bone-resorbing factor which is secreted from tumor tissue and cells of a hypercalcemic patient with malignant melanoma who had no skeletal metastases and undetectable plasma immunoreactive parathyroid hormone. This bone-resorbing factor continues to be secreted after 16 months, both by tumor cells in continuous culture and by tumor serially passed in nude mice. Potent bone-resorbing activity also has been extracted from the patient's frozen tumor. This factor is a peptide of approximately 13,000 daltons; it can be distinguished from parathyroid hormone and osteoclast-activating factor (factors which have been implicated in the genesis of cancer-associated hypercalcemia), and its activity is not due to stimulation of cyclic AMP or prostaglandin synthesis by bone cells. Our central objectives are to purify this factor, to establish its chemical structure and to develop sensitive radioassays for its measurement. We will purify this factor to homogeneity, and determine its chemical structure. We will develop monoclonal antibodies which will be used to purify the peptide and to serve in molecular biological studies. In addition, we will prepare sufficient material to initiate immunization programs to intact animals expecting that the polyclonal immune response in vivo is more likely to yield high-affinity immunological reagents useful in developing radioimmunoassays for measurement of MRF during purification and in blood and body fluids. Achieving these goals will have clinical impact by providing chemical characterization of this factor and developing immunological reagents for its measurement. These steps are necessary to determine the role of this factor in cancer-associated hypercalcemia and to understanding the pathophysiology of this syndrome.